It is just a question of organizing the facts to test in a convenient way that can be tested without too much muss or fuss. The following untested validation function should do it: 
# Tests the start_hour, start_minute, end_hour, and end_minute
# pulldowns to verify that a consistent set of them have been
# specified, and that the start time (if specified) is before
# the end time.
sub validate_date_pulldowns {
  if (defined(param('end_hour'))) {
    return 0 unless defined(param('start_hour'));
    return 0 if param('start_hour') > param('end_hour');
  }
  if (defined(param('start_minute'))) {
    return 0 unless defined(param('start_hour'));
  }
  if (defined(param('end_minute'))) {
    return 0 unless defined(param('start_minute'));
    return 0 unless defined(param('end_hour'));
    return 0
      if param('start_hour') == param('end_hour') and
         param('start_minute') > param('end_minute');
  }
  return 1;
}

[download]
It would be easy to extend this code sample to add in support for start/end seconds as well.

Update: Note that the key idea is my strategy of turning many combinations of conditions into a set of "guard conditions" on which you exit early. I've found that this strategy often converts deep nested decision trees into a series of fairly straightforward decisions (as above).

In reply to Re: Testing for 16 possible combinations with 4 pull-down menus by tilly
in thread Testing for 16 possible combinations with 4 pull-down menus by softcotton

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.